Radiobiology is the study of how ionizing radiation interacts with living things. This document provides an overview of different types of ionizing radiation including electromagnetic radiation like x-rays and gamma rays, and particulate radiation like electrons, protons, alpha particles, and heavy ions. It describes the physics of how this radiation is absorbed and can cause excitation or ionization in biological materials. Specifically, it notes that x-rays and gamma rays are indirectly ionizing since they produce fast-moving electrons upon absorption, while particulate radiations can directly ionize materials. Around 10,000 to 20,000 cases of lung cancer per year in the US are attributed to alpha particles from radon gas in homes.

1. Introduction to
Radiobiology for
Radiation Oncologists
By
Dina Barakat

2. What is Radiobiology?
 Radiobiology is the study of the action of ionizing radiations on living things.
As such, it inevitably involves a certain amount of radiation physics.
 The purpose of this introduction is to present, in summary form and with a
minimum of mathematics, a listing of the various types of ionizing radiations
and a description of the physics and chemistry of the processes by which
radiation is absorbed.

3. Types
Radiation

5. Excitation vs
Ionisation
 The absorption of energy from radiation in
biologic material may lead to excitation or to
ionization.
 The raising of an electron in an atom or
molecule to a higher energy level without
actual ejection of the electron is called
excitation.
 If the radiation has sufficient energy to eject
one or more orbital electrons from the atom or
molecule, the process is called ionization, and
that radiation is said to be ionizing radiation.

6. Ionizing radiation
 The important characteristic of ionizing radiation is the localized release of
large amounts of energy.
 The energy dissipated per ionizing event is about 33 eV, which is more than
enough to break a strong chemical bond; for example, the energy associated
with a C=C bond is 4.9 eV.
 For convenience, it is usual to classify ionizing radiations as either
electromagnetic or particulate.

7. Ionizing radiation
Electromagnetic
X-rays
Gamma rays
Particulate
Electrons
Protons
Alpha particles
Heavy ions (Fe, C, etc)

8. Basic
Concepts

9. X-rays
 Type of radio-magnetic ionizing radiation.
 X-rays are produced extranuclearly.
 In practical terms, this means that x-rays are produced in
an electrical device that accelerates electrons to high
energy and then stops them abruptly in a target usually
made of tungsten or gold. Part of the kinetic energy (the
energy of motion) of the electrons is converted to x-rays.
 They can be thought of either electrical waves or packets
of energy.

10. Gamma rays
 They are form of radio-magnetic ionizing radiation
 γ-rays are produced intranuclearly
 γ-rays are emitted by radioactive isotopes; they represent
excess energy that is given off as the unstable nucleus
breaks up and decays in its efforts to reach a stable form.
Natural background radiation from rocks in the earth also
includes γ-rays.

11. Electrons
 A form of particulate ionising radiation
 Electrons are small, negatively charged particles that can
be accelerated to high energy to a speed close to that of
light by means of an electrical device, such as a betatron
or linear accelerator. They are widely used for cancer
therapy.

12. Protons
 A form of particulate (particle) ionising radiation.
 Protons are positively charged particles and are relatively
massive, having a mass almost 2,000 times greater than
that of an electron. Because of their mass, they require
more complex and more expensive equipment, such as a
cyclotron, to accelerate them to useful energies, but they
are increasingly used for cancer treatment in specialized
centers because of their favorable dose distribution

13. Alpha particles
 They are form of particulate ionising radiation
 α-Particles are nuclei of helium atoms and consist of two protons and two
neutrons in close association.
 They have a net positive charge and, therefore, can be accelerated in large
electrical devices similar to those used for protons.
 α-Particles are also emitted during the decay of heavy, naturally occurring
radionuclides, such as uranium and radium.
 α-Particles are the major source of natural background radiation to the
general public. Radon gas seeps out of the soil and builds up inside houses,
where, together with its decay products, it is breathed in and irradiates the
lining of the lung. It is estimated that 10,000 to 20,000 cases of lung cancer
are caused each year by this means in the United States, mostly in smokers.

14. Neutrons
 Form of particulate ionising radiation
 Neutrons are particles with a mass similar to that of protons, but they carry no
electrical charge.
 Because they are electrically neutral, they cannot be accelerated in an electrical
device. They are produced if a charged particle is accelerated to high energy and
then made to impinge on a suitable target material.
 Neutrons are also emitted as a by-product if heavy radioactive atoms undergo
fission; that is, a split to form two smaller atoms. Consequently, neutrons are
present in large quantities in nuclear reactors and are emitted by some artificial
heavy radionuclides.
 They are also an important component of space radiation and contribute
significantly to the exposure of passengers and crews of high-flying jetliners.

15. Heavy charged particles
 They are form of particulate ionising radiation
 Heavy charged particles are nuclei of elements, such as
carbon, neon, argon, or even iron, that are positively
charged because some or all of the planetary electrons
have been stripped from them. To be useful for radiation
therapy, they must be accelerated to energies of
thousands of millions of volts and, therefore, can be
produced in only a few specialized facilities, although the
number of such centers is increasing.

16. Absorption of x-rays
 Radiation may be classified as directly or indirectly ionizing.
 All of the charged particles previously discussed are directly ionizing;
that is, provided the individual particles have sufficient kinetic
energy, they can disrupt the atomic structure of the absorber through
which they pass directly and produce chemical and biologic changes.
 Electromagnetic radiations (x- and γ-rays) are indirectly ionizing.
They do not produce chemical and biologic damage themselves, but
when they are absorbed in the material through which they pass, they
give up their energy to produce fast-moving charged particles that in
turn are able to produce damage.

21. Summary
 X- and γ-rays are indirectly ionizing; the first step in their absorption is the
production of fast recoil electrons.
 Neutrons are also indirectly ionizing; the first step in their absorption is the
production of fast recoil protons, α-particles, and heavier nuclear fragments.
 Biologic effects of x-rays may be caused by direct action (the recoil electron directly
ionizes the target molecule) or indirect action (the recoil electron interacts with
water to produce an OH·, which diffuses to the target molecule).
 About two-thirds of the biologic damage by x-rays is caused by indirect action (i.e.,
involving free radicals), and this component of the biologic damage can be modified
by chemical protectors.
 Chemical protectors are less effective with high-LET radiations where most biologic
damage is a result of the direct effect.

23.  I-1) Which one of the following statements concerning the
interaction of photons with matter is CORRECT?
A. The probability of the photoelectric effect decreases
with the atomic number of the absorber
B. The predominant interaction of 10 keV photons with
soft tissue is the Compton process
C. In the Compton process, the energy of the scattered
photon is less than that of the incident photon
D. Pair production occurs for photons with energies less
than 1.02 MeV E. There is only partial absorption of the
energy of the incident photon in the photoelectric effect

24.  The approximate minimum photon energy required to
cause ionization is:
A. 10-25 eV
B. 100-250 eV
C. 1-2.5 keV
D. 10-25 keV
E. 100-250 keV

25.  2-Which of the following X-ray interactions with matter is
most important for - producing high-contrast diagnostic
radiographs?
A. Compton process
B. pair production
C. photoelectric effect
D. nuclear disintegration
E. coherent scattering

26.  3-Which of the following pairs of photon energy and
predominant atomic interaction at that energy is correct?
A. 1 keV – pair production
B. 50 keV – triplet production 9
C. 100 keV – Compton process
D. 2 MeV – photoelectric effect

27.  4-Approximately 10,000-20,000 cases of lung cancer each
year in the United States are attributed to alpha-particles
produced by:
A. Nuclear weapon testing
B. Decrease in the ozone layer
C. Radon gas
D. Chemical contamination

28.  5-In pair production, which of the following is true?
A. The incident photon is scattered with reduced energy.
B. Annihilation photons always have an energy of 0.51
MeV each.
C. A pair of orbital electrons are ejected from the atom.
D. Two positrons are emitted at 180 degrees.
E. It cannot occur if the photon energy is above 1.02 MeV.

29.  6-Directly ionizing radiation includes all of the following
EXCEPT:
A. Electrons
B. Positrons
C. Alpha particles
D. Neutrons
E. Betas.

30.  7-When a live human cell is irradiated by gamma-rays,
which one of the following events may eventually cause
the most of the damage to DNA.
A. Absorption of radiation energies by the chemical bonds
in the DNA molecules.
B. Ionization and excitation on atoms within the DNA
structure.
C. Ionization and excitation on atoms within the histones
that bound to DNA.
D. Ionization and excitation of the water molecules that
surround DNA.
E. Direct damage to the lipids that may later oxidize DNA.

31.  Answers:
1. C
2. A
3. C
4. C
5. C
6. B
7. D
8. D

32. Any
Questions?